Leak down testers

[H2OPERF]

My question to all is how they can be used on a wide variety of bore sizes with the same orifice size in the gage assembly with people using different pressures and have the numbers really mean anything.. I you tested your motor from new and allways used the same unit yes they would give you a standard to go from for the life of the engine.. MY thoughts are...If you do a 125cc bike cylinder with a .040 orifice in the tester and 80 psi (aviation Standard) on the regulator then do the same on a 500cc omc cylinder and get 80/75 wouldn't this means the amount of damage to give this amount of leakage to be the same in both cylinders? so wouldn't this correlate to the 125 bore being junk in the contact area compared to the 500cc bore having the same damage spread out over way more circumfrance...some one school me if im wrong in my thinking..Dave

[gfinch]

Copy and paste....
Leakage is given in wholly arbitrary percentages but these “percentages” do not relate to any actual quantity or real dimension. The meaning of the readings is only relative to other tests done with the same tester design. Leak-down readings of up to 20% are usually acceptable. Leakages over 20% generally indicate internal repairs are required. Racing engines would be in the 1-10% range for top performance, although this number can vary. Ideally, a baseline number should be taken on a fresh engine and recorded. The same leakage tester, or the same leakage tester design, can be used to determine wear.
In the United States, FAA specifications^[1] state that engines up to 1,000 cu in (16 L) engine displacement require an 0.040 in (1.0 mm) orifice diameter, 0.250 in (6.4 mm) long, 60-degree approach angle. The input pressure is set for 80 psi (550 kPa), and 60 psi (410 kPa) minimum cylinder pressure is the accepted standard.
While the leak-down tester pressurizes the cylinder, the mechanic can listen to various parts to determine where any leak may originate. For example, a leaking exhaust valve will make a hissing noise in the exhaust pipe while a head gasket may cause bubbling in the cooling system.

They forgot intake valve, air coming out a neighboring cylinder or out the side of the block/head interface. I'm sure there are others but don't care.

I were a grease monkey 45 years....it's a tool to use and decide which way to go. I have a low compression reading, now to find out where it's leaking.
I rarely did compression tests and recorded values. What I did do was disable ignition so the engine wouldn't start, cranked it over and listened to the beat. In front of the consumer in the driveway. If it regularly skipped a beat it had low compression. Then the decision was made to either go into further diagnosis $$$ or the consumer drove away. I didn't make any $ but I didn't waste time either on something that they already weren't going to spend $$$.
Now on a cylinder you can have different leakages at different points/heights in the cylinder. I had 22% at the top and 75% about 1/2" above the exhaust port in one cylinder.
If you have excessive leakage you should tear down, inspect then repair.
Butt, I've been wrong before and will be again.

[H2OPERF]

Thanks Gary, That kinda agrees with my thoughts, reason i posted is a friend just did a small motor 20hp with the same tester we all use from the aircraft industry and i told him the numbers didnt mean shizit and it started into a conversation (argument ) that the %numbers work for any engine and they really don't..Yes it will tell you if you have a pile of junk or a perfect seal but the % is not equal on all cylinder sizes. Most of the testers are made for aircraft use and they have big ass pistons(lycoming bore is 5.125) and if everyones using a certified gage and 80 psi the numbers mill mean something since most of the bores are the same on the 4 and six cylinder engines, now we use this on a 3.125 bore 10% isnt relative and change the pressure u do it at and it all goes out the window again. I see some automotive ones use 100psi wouldn't that refill the cyl faster than 80psi with the same size orifice .. Just my Thought s Dave

[Chaz]

It's funny how different sect's of the reciprocating I.C. workhorse value monitoring tools very differently .
If we were to follow the FAA or SAE pocket protector ( tape on nose bridge of glasses optional) geeks , everyone would need to learn to convert percent to decimal to figure out that their 80 psi base that held 72 , pizzed out 8 , was really 10% leakage .
Those of us that are plumb dumb , would say ... that's the dumbest thing I ever seen. Those of us that are some dumb would say , well my motor makes way more than 80 psi and some problems might not really show up until higher pressure is reached , but I like the low pressure , cuz the piston is less likely to cock in the bore ... or try to let the breaker bar slap me in the head .
Since 100 psi gauges are made by everyone and we are only dealing only with measuring the loss of trapped air in a stationary position at this point . I need to brush up on my Latin . "Per Centum = for each one hundred " ... Makes it easy as penny's to a dollar , to cipher and remember without needing pencil , paper or I-Pad .. whatever number there up to now ..

Orifice size .. the standard for flowing a fuel pressure regulator in the nitrous world is to use a Holley .073 jet . (which has a .079 diam hole)
I cant tell you the "people who learned me" .. boy what size jet you got in there .. a 79 sir .. that thing aint gonna run .. ya need you a 73 to git it rite .. Ohhh , OK .
I actually went as small as I had which was a 67 and as big as I had which was a 98 and it made no difference at all . My conclusion was that it's just an arbitrary number to create pressure differential between the measuring devise and the open atmosphere . As long as it's not so small that it deadheads , or so large that the pump can't keep up .. it will give you a repeatable , quantifiable number . just as the orifice in a leak tester provides .

Me , I'm more of a compression gauge guy . The first three pumps gives a pretty good indication of where you stand by , needle swing , starter grunt and then psi achieved . I have two leak testers and a bore scope .. but I see those as more of a back-up deal .

The real deal guys with vacuum pump's , dry sumps , etc. Just look at a crankcase vacuum gauge . When they know the bypass is set at -17 in/hg and all of a sudden it's went +5 positive , it's best to pull it behind the wall before it goes +150 ..

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Dave ,
Next time we talk , we gotta cover how you get to 100% V.E. @ 10,000 rpm .... Seems a little past torque peak to me ...

nebba mine , I'md figgerd id oud .. de twain iz nut dunn rekkin yed-t ..

[phillnjack]

Nice one chaz.

[gfinch]

Yep.. 3 pumps and done. OMG did I say that!

[roadkill636]

Wheres Balzy when we need him?

[Krazymaan]

Thats why he takes so many naps during the day. Three pumps and he's done.

[H2OPERF]

Compression gage for me when I build it then again when the tachs not reading right.. I only use the differential tester when I"m filling out a log book and using a prop to hold it back. Chaz my mota only runs good 1/2 of the time since the cheap ass omc vac pump only works on the up stroke. Dave

[Chaz]

Compression gage for me when I build it then again when the tachs not reading right.. I only use the differential tester when I"m filling out a log book and using a prop to hold it back. Chaz my mota only runs good 1/2 of the time since the cheap ass omc vac pump only works on the up stroke. Dave

Yes , but that "three eight fifty four" creates quite a following ...

[H2OPERF]

Yes it does, be the only one without all those extra parts after next year so i'm told.. Dave

[baja200merk]

DAve you have to put it on the boat to wear it out

Yes , but that "three eight fifty four" creates quite a following ...

Three eight eighty four with 3.4 crank is almost 3.5L

[H2OPERF]

I know I know... this move has killed me, freakin county bs permits on the old house took 6mos couldnt sell it w/open permits and they work at sloth speed . Still have the HO on the flats running that counts.

[badbradviper]

I was told a long time ago that you need to buy a tester compression or leak down and use it on as manny of the same engine as you can. in order to know what the readings mean not all gauges read the same.